1. Field of the Invention
The present invention relates to multi-channel audio and particularly to the delivery of high quality and distortion-free multi-channel audio in an enclosure.
2. Description of the Background Art
The inventors have recognized that the acoustics of an enclosure (e.g., room, automobile interior, movie theaters, etc.) play a major role in introducing distortions in the audio signal perceived by listeners.
A typical room is an acoustic enclosure that can be modeled as a linear system whose behavior at a particular listening position is characterized by an impulse response, h(n) {n=0, 1, . . ., N−1}. This is called the room impulse response and has an associated frequency response, H(ejω). Generally, H(ejω) is also referred to as the room transfer function (RTF). The impulse response yields a complete description of the changes a sound signal undergoes when it travels from a source to a receiver (microphone/listener). The signal at the receiver contains consists of direct path components, discrete reflections that arrive a few milliseconds after the direct sound, as well as a reverberant field component.
It is well established that room responses change with source and receiver locations in a room. A room response can be uniquely defined for a set of spatial co-ordinates (xi, yi, zi). This assumes that the source (loudspeaker) is at origin (0, 0, 0) and the receiver (microphone or listener) is at the spatial co-ordinates, xi, yi and zi, relative to a source in the room.
Now, when sound is transmitted in a room from a source to a specific receiver, the frequency response of the audio signal is distorted at the receiving position mainly due to interactions with room boundaries and the buildup of standing waves at low frequencies.
One mechanism to minimize these distortions is to introduce an equalizing filter that is an inverse (or approximate inverse) of the room impulse response for a given source-receiver position. This equalizing filter is applied to the audio signal before it is transmitted by the loudspeaker source. Thus, if heq(n) is the equalizing filter for h(n), then, for perfect equalization heq(n){circle around (x)}h(n)=δ(n); where {circle around (x)} is the convolution operator and δ(n) is the Kronecker delta function.
However, the inventors have realized that at least two problems arise when using this approach, (i) the room response is not necessarily invertible (i.e., it is not minimum phase), and (ii) designing an equalizing filter for a specific receiver (or listener) will produce poor equalization performance at other locations in the room. In other words, multiple-listener equalization cannot be achieved with a single equalizing filter. Thus, room equalization, which has traditionally been approached as a classic inverse filter problem, will not work in practical environments where multiple-listeners are present.
Furthermore, it is required that for real-time digital signal processing, low filter orders are required. Given this, there is a need to develop a system and a method for correcting distortions introduced by the room, simultaneously, at multiple-listener positions using low filter orders.